1. Field
This disclosure is generally related to solar cells. More specifically, this disclosure is related to a solar cell based on a tunneling junction structure that uses a dielectric material (e.g. silicon oxide) to form a tunneling layer.
2. Related Art
The negative environmental impact caused by the use of fossil fuels and their rising cost have resulted in a dire need for cleaner, cheaper alternative energy sources. Among different forms of alternative energy sources, solar power has been favored for its cleanness and wide availability.
A solar cell converts light into electricity using the photoelectric effect. There are many solar cell structures and a typical solar cell contains a p-n junction that includes a p-type doped layer and an n-type doped layer. In addition, there are other types of solar cells that are not based on p-n junctions. For example, a solar cell can be based on a metal-insulator-semiconductor (MIS) structure that includes an ultra-thin dielectric or insulating interfacial tunneling layer situated between a metal or a highly conductive layer and a doped semiconductor layer.
In a p-n junction based solar cell, the absorbed light generates carriers. These carriers diffuse into the p-n junction and are separated by the built-in electric field, thus producing an electrical current across the device and external circuitry. An important metric in determining a solar cell's quality is its energy-conversion efficiency, which is defined as the ratio between power converted (from absorbed light to electrical energy) and power collected when the solar cell is connected to an electrical circuit.
To increase the conversion efficiency, a solar cell structure should allow the photon-generated carriers to effectively transport to the electrode. One of the important paths for carrier loss is the recombination of minority carriers at the cell surfaces. Hence, excellent surface passivation is important for solar cell performance because it directly impacts the open circuit voltage (Voc). Note that a good Voc implies a good temperature coefficient, which enables a better solar cell performance at higher temperatures. For homojunction solar cells, minority-carrier recombination at the solar cell surfaces due to the existence of dangling bonds can significantly degrade the surface passivation. In addition, the relatively thick, heavily doped emitter layer, which is formed by dopant diffusion, typically results in worse minority-carrier recombination as well as drastically reduces short-wavelength response. Heterojunction solar cells, such as Si heterojunction (SHJ) solar cells tend to have better surface passivation due to an inherent bandgap offset between an amorphous-Si (a-Si) layer and a crystalline-Si (c-Si) base layer, which reduces the surface recombination velocity by creating a barrier for majority carriers. The a-Si layer also passivates the surface of the c-Si base layer by repairing the existing Si dangling bonds through hydrogenation. However, the a-Si layer that usually forms the emitter is heavily doped, thus resulting in degraded surface passivation and reduced short-wavelength response. To mitigate the increased minority-carrier surface recombination, an intrinsic a-Si layer can be inserted between the heavy doped a-Si layer and the c-Si base layer.
In MIS-based solar cells, an insulating layer, such as silicon oxide with a certain fixed interface charge, is inserted between a metal layer and a semiconductor layer (such as a doped c-Si layer) and induces an inversion layer in the doped c-Si. The built-in surface field at the interface between the inversion layer and the c-Si allows charges to be separated and collected as the minority carriers travels through the highly conductive space-charge region by tunneling through the ultra thin insulating (or oxide) layer. Extremely low surface recombination velocity can be achieved via the surface passivation of the inversion layer and the low defect density states of the inversion “emitter” layer. To avoid introducing high series resistance associated with carrier tunneling through a thick dielectric layer, the tunnel oxide layer needs to be less than 20 or 30 angstroms.
FIG. 1A presents a diagram illustrating an exemplary metal-insulator-semiconductor (MIS) solar cell (prior art). MIS solar cell 100 includes a silicon substrate layer 102 doped with one type of dopant, a thin insulating layer 104, a top metal grid 106, and a bottom metal contact layer 108. Arrows in FIG. 1A indicates incident sunlight.
Although it is possible to achieve good efficiency with MIS-based solar cells on a small scale, it is difficult to scale up the MIS-base solar cells to manufacturable sizes because the poor conductivity of the inversion layer requires a dense metal grid for current collection. In addition, the Cesium (Cs) doped in the oxide layer, which is introduced to induce the inversion layer, is not stable.
On the other hand, all ultra-high-efficiency mono-crystalline Si-based solar cell structures are specifically designed to enable high Voc through excellent surface passivation. The current record holder of solar cell efficiency is a solar cell based on an interdigited back-contact (IBC) structure as described in U.S. Pat. No. 4,927,770. In the IBC-based solar cell, good Voc is achieved with a front- and back-surface oxide passivation as well as a rear interdigited emitter and base point contacts. A further improvement made for the IBC structure is shown in U.S. Pat. No. 7,737,357, where the rear interdigited emitter and base contacts are replaced with doped a-Si contacts formed on a tunnel SiO2 layer. This approach results in superior surface passivation provided by the tunnel oxide and the a-Si induced heterojunction field, which led to a Voc greater than 700 mV.
U.S. Pat. No. 5,705,828 discloses a double-sided heterojunction solar cell, which is also a high-efficiency solar cell based on excellent surface passivation. A double-sided heterojunction solar cell can achieve high efficiency with higher open circuit voltage (Voc), such as greater than 715 mV, by implementing a high-quality intrinsic a-Si interface layer. The intrinsic a-Si layer reduces the number of surface dangling bonds and provides an inherent heterojunction bandgap offset, which creates a favorable heterojunction field for reducing leakage current.
Other approaches to obtain high-efficiency solar cells by improving surface passivation have been proposed. U.S. Pat. No. 7,164,150 further describes a method for suppressing carrier recombination at the interface between the c-Si base layer and the intrinsic a-Si layer by introducing carbon dioxide (CO2) gas during the deposition of the intrinsic a-Si. Voc of the solar cell can be increased by tuning the processing condition during the a-Si deposition. U.S. Pat. No. 4,404,422 describes a MIS-based solar cell that implements an ultra-thin tunneling oxide layer to enable very low interface density states without increasing the series resistance associated with tunneling through the dielectric layer. Different variations of the MIS-based solar cell structures are described in U.S. Pat. Nos. 4,828,628 and 4,343,962. These solar cells gain good performance by inducing an inversion layer in the c-Si base. The use of the oxide layer and the inversion-induced emitter lead to the surface passivation and the reduction of surface defect states, respectively. The combination of these effects results in an excellent minority-carrier recombination velocity.
However, further improvement is still needed to obtain solar cells with even better performance as well as the potential to achieve solar cells with efficiency greater than 23%. An even higher Voc (higher than 750 mV) is a crucial condition for an ultra-high performance solar cell with an extremely low temperature coefficient (less than 0.25%/° C.). To accomplish such a high Voc, it is desirable to obtain a very low defect-interface-state density (Dit), such as less than 1×1011/cm2.